Beautiful work Mr Avaiojet. Makes my efforts look weak in comparison! I use permanent markers. I also use a flexi ruler ( very soft plastic) to follow curves over wings etc. I have found a clear plastic small/medium 45 , or 30 degree set square great for really tight spots. I usually just make up the panel lines, sometimes I'll try to copy the real thing. Although rub proof when dry, not 100% fuel/oil proof - so I give several coats of nail varnish overlines. OR spray the entire plane with 2-3 coats automotive clearcoat lacquer. This is reasonably resilient on the model. (It will slightly dull if raw fuel gets on it for more than a few seconds, which if careful shouldn't really happen anyway - yeah right! ) Caution one or two very very light dusting coats over ink which has set overnight before spraying 2-3 coats of clear. Do this is warm conditions. Couple of piccies below - bit rough but effective. Amazing how the eye fills in the missing bits!